1. Field of the Invention
This invention relates to a semiconductor laser device which oscillates laser light with an oscillation wavelength in the visible region, especially, a high quality semiconductor laser device which can be readily produced by molecular beam epitaxy.
2. Description of the Prior Art
In recent years, single crystal thin film growth techniques such as molecular beam epitaxy (MBE) and metal organic-chemical vapor deposition (MO-CVD), have rapidly improved. By these growth techniques, it is possible to obtain epitaxial growth layers of extreme thinness, on the order of 10 .ANG.. Due to the progress in these crystal growth techniques, it is possible to make laser devices based on device structures having very thin layers, which could not be easily manufactured by conventional liquid phase epitaxy. A typical example of these laser devices is the quantum well (QW) laser, in which the active layer has a thickness of 100 .ANG. or less resulting in the formation of quantum levels therein, whereas the active layer of the conventional double-heterostructure (DH) laser has a thickness of several hundreds of angstroms or more. Thus, this QW laser is advantageous over the conventional DH laser in that the threshold current level is reduced, the temperature characteristics are excellent, and the transient characteristics are excellent. This has been reported by W. T. Tsang, Applied Physics Letters, vol. 39, No. 10, p. 786 (1981); N. K. Dutta, Journal of Applied Physics, vol. 53, No. 11 p. 7211 (1982); and H. Iwamura, T. Saku, T. Ishibashi, K Otsuka, Y. Horikoshi, Electronics Letters, vol. 19, No. 5, p. 180 (1983).
As mentioned above, by the use of single thin crystal film growth techniques such as MBE and MO-CVD, it is now possible to put high quality semiconductor lasers having a new multiple-layered structure into practical use.
A typical structure for conventional QW lasers is the AlGaAs laser with a graded-index separate-confinement heterostructure (GRIN-SCH). The AlAs mole fraction (i.e., x) in an Al.sub.x Ga.sub.1-x As mixed crystal in the active region of these GRIN-SCH lasers is shown in FIG. 5. These GRIN-SCH lasers have a cladding layer 1, an optical guiding layer 2, a quantum well active layer 3, an optical guiding layer 4 and a cladding layer 5 in that order, wherein the recombination of carrier arises in the quantum well active layer 3, resulting in laser oscillation, which causes laser light. The laser light is guided by the optical guiding layers 2 and 4.
In order that the laser light is guided by the optical guiding layers 2 and 4 in the GRIN-SCH lasers, sufficient differences .DELTA.n in the refractive index between the quantum well active layer 3 and the optical guiding layer 2 and between the quantum well active layer 3 and the optical guiding layer 4 must be created in such a manner that the refractive index of each of the optical guiding layers 2 and 4 is greater than that of the quantum well active layer 3 positioned between these optical guiding layers 2 and 4. For this reason, sufficient differences in the AlAs mole fraction (i.e., x) must be created in the direction from the quantum well active layer 3 to each of the optical guiding layers 2 and 4. On the other hand, when a visible light semiconductor laser device oscillating laser light with a short oscillation wavelength is produced with the said GRIN-SCH laser structure, the carrier must be sufficiently confined within the quantum well active layer 3, which causes the necessity of a sufficiently high quantum well barrier.
FIG. 6 shows the dependence of the relative height of both the conduction-band edge and the valence-band edge in an Al.sub.x Ga.sub.1-x As mixed crystal on the AlAs mole fraction (i.e., x) in the Al.sub.x Ga.sub.1-x As mixed crystal. The height of the conduction-band edge and the valence-band edge shown in FIG. 6 is determined based on the experimental data that the band discontinuities of the conduction-band and the valance-band are distributed with a proportion of 60% and 40% at the hetero-interface between the two different Al.sub.x Ga.sub.1-x As mixed crystals.
It can be seen from FIG. 6 that when x is smaller than 0.45 (i.e., x&lt;0.45), the conduction-band edge is the direct transition region at the .GAMMA. point and linearly increases with an increase in x, and that when x is greater than 0.45 (i.e., x&gt;0.45), the X point of the indirect transition region forms the conduction-band edge and gradually decreases with an increase in x. These phenomena are referred to in the following references: H. Kroemer, The 2nd International Conference on Modulated Semiconductor Structures V-(24), p. 797(1985); H. C. Casey and M. B. Panish, Heterostructure Lasers (Acedemic Press, 1978); M. O. Watanabe et al., J. Appl. Phys. 57, p. 5340(1985); D. Arnold et al., Appl. Phys. Lett. 44, p. 1237(1984).
Thus, in order to create a sufficiently high quantum well barrier, it is necessary to make the AlAs mole fraction (i.e., x) sufficiently high in the portion of each of the optical guiding layers 2 and 4 in the vicinity of the quantum well active layer 3 positioned between these optical guiding layers 2 and 4. Moreover, in order to effectively achieve the confinement of laser light within the quantum well active layer 3, it is necessary for the AlAs mole fraction, which is higher than that of the above-mentioned portion of each of the optical guiding layers 2 and 4, to be set in the portion of each of the optical guiding layers 2 and 4 in the vicinity of the cladding layers 1 and 5 positioned outside of the optical guiding layers 2 and 4. For this purpose, it is necessary to use a mixed crystal in the indirect transition region, the AlAs mole fraction (i.e., x) of which is 0.45 or more, for the optical guiding layers 2 and 4. However, when such a mixed crystal in the indirect transition region is used for the optical guiding layers 2 and 4, the following problems arise:
FIG. 7 shows the conduction-band energy in the active region of a GRIN-SCH laser in which the optical guiding layers 2 and 4 are formed by an Al.sub.x Ga.sub.1-x As mixed crystal in which x is greater than 0.45 (i.e., x&gt;0.45), wherein the optical guiding layers 2 and 4, respectively, have an AlAs mole fraction x with gradual changes from 0.7 to 0.45 in the direction from the outside of each of the optical guiding layers 2 and 4 to the GaAs active layer 3 positioned between the optical guiding layers 2 and 4. FIG. 7 indicates that when electrons are injected from the optical guiding layers 2 and 4 into the active layer 3, the conduction-band edge of each of the optical guiding layers 2 and 4 becomes higher in the direction from the outside of each of the optical guiding layers 2 and 4 to the GaAs active layer 3, so that these optical guiding layers 2 and 4 function as a barrier against the electrons. This means that when an Al.sub.x Ga.sub.1-x As mixed crystal having an AlAs mole fraction x (wherein x&gt;0.45) is used for the optical guiding layers 2 and 4, the greater difference .DELTA.n in the refractive index between the active layer 3 and each of the optical guiding layers 2 and 4 is set in order to effectively confine laser light within the active layer 3, the higher barrier is created against the electrons to be injected, which causes the lowering of the electron injection efficiency. Therefore, so long as an Al.sub.x Ga.sub.1-x As mixed crystal in the indirect transition region is employed for the optical guiding layers, it is difficult to simultaneously achieve the complete confinement of light within the active layer and the highly efficient injection of carrier from the optical guiding layers into the active layer. FIG. 8 shows a sectional view of the GRIN-SCH laser device shown in FIG. 5. The GRIN-SCH laser device is produced as follows: On an n-GaAs substrate 10, n-Al.sub.x Ga.sub.1-x As (x=0.7) cladding layer 1, non-doped Al.sub.x Ga.sub.1-x As GRIN optical guiding layer 2 in which the AlAs mole fraction x is gradually decreased from 0.7 to 0.3, non-doped GaAs active layer 3, non-doped Al.sub.x Ga.sub.1-x As GRIN optical guiding layer 4 in which the AlAs mole fraction x is gradually increased from 0.3 to 0.7, p-Al.sub.x Ga.sub.1-x As (x=0.7) cladding layer 5, a p-GaAs cap layer 6, and an Al.sub.0.5 Ga.sub.0.5 As current blocking layer 7 are successively grown. Thereafter, the current blocking layer 7 is subjected to a mask-etching treatment to form a stripe having a width of 5 .mu.m, resulting in a striped semiconductor laser device.